Vladimir Putin has accused Hillary Clinton, the US secretary of state, of fomenting an increasingly vociferous opposition movement in Russia, threatening to derail the two countries' fragile resetting of relations.

The accusation builds on months of Russian statements and media coverage blaming popular uprisings around the Arab world on western scheming. It comes as Washington and Moscow tussle over a host of disagreements, from missile defence to Syria.

Speaking to supporters on Thursday, Putin accused Clinton of giving "the signal" to opposition leaders, who are expected to gather with tens of thousands of supporters for a protest on Saturday. He rejected Clinton's repeated criticism of a parliamentary vote last weekend that gave Putin's United Russia party nearly 50% of the vote amid widespread reports of fraud.

"[Opposition leaders] heard the signal and with the support of the US state department began active work," Putin said during a meeting of the All-Russia People's Front, a new political movement set up to support his presidential candidacy in a 4 March election.

"We are all grownups here. We all understand the organisers are acting according to a well-known scenario and in their own mercenary political interests," he said.

Clinton raised the issue of Russia's elections again on Thursday during a visit to Brussels. "Human rights is part of who we are," she said, after Putin's comments emerged. "And we expressed concerns that we thought were well founded about the conduct of the elections.

"We are supportive of the rights and aspirations of the Russian people to be able to make progress and realise a better future for themselves."

Russian opposition leaders have begun to express concern about how the Kremlin will react to Saturday's protest, spawned by growing outrage at multiple examples of electoral fraud. Nearly 30,000 people have indicated their intention to join the protest on Moscow's Revolutionary Square via Facebook. Protests have been organised in more than 80 cities around the country.

The Kremlin has stepped up the security presence in the capital, with more than 50,000 police and 2,000 interior troops patrolling the streets. Water cannon and helicopters have also been seen in Moscow.

"No one wants chaos," Putin said, adding that most Russians did not want a repeat of the overthrowing of governments in nearby Kyrgyzstan and Ukraine.

Putin has often accused the west of meddling in Russian affairs, a tested tactic to deflect attention away from the country's problems. "We are required to protect our sovereignty," he said. "We will have to think about strengthening the law and holding more responsible those who carry out the task of a foreign government to influence internal political processes."

Putin made a similar pronouncement a week before the vote, prompting a campaign against Golos, an independent election monitor that gets foreign grants.

Putin's statements marked the first time he has openly acknowledged liberal opposition to his rule. "We must carry out a dialogue with the opposition-minded, and give them the chance to use their constitutional right to demonstrate," he said. But he warned that illegal means of protest would be punished. "If someone breaks the law, then the organs of power and keepers of order must demand the law be followed."

The liberal opposition has carried out a long campaign to win the right to demonstrate, and has almost always been denied. City authorities gave permission for Saturday's rally, but warned that the permit only allowed 300 people to gather. The mayor's office was holding talks with opposition leaders in a bid to move the protest away from Revolution Square, a stone's throw from the Kremlin.

President Dmitry Medvedev also mentioned the growing protest movement during a visit to Prague on Thursday. "People must have the possibility to say their opinion, that's normal," he said. "The most important thing now is to calm nerves and allow the parliament to begin working."

Activists, organising via the internet, began spreading information on how to behave during Saturday's protest and what to do in case of arrest. A spokesman for VKontakte, the Russian version of Facebook, said the company had received a request from the Federal Security Service to shut down groups related to the protests, but declined to follow it.

While Putin has defended the election result, he also appeared to try to distance himself from United Russia, the ruling party and target of protesters' wrath. He told members of the Popular Front – which critics call a "rebranded" United Russia – the party was pressuring deputies from the new group to use their parliamentary mandates in favour of United Russia.

"I relate to United Russia with very fond feelings – it's an organisation that I, in my time, created, but I ask you not to give in to pressure," he said.

United Russia was created in 2001 with the sole purpose of supporting Putin's agenda. The Popular Front was created earlier this year to do the same thing, ahead of next year's presidential vote.

Infamy, infamy

Vladimir Putin, Russia's beleaguered prime minister, has joined a motley crew of dictators and assorted autocrats, grand panjandrums and political buffoons who blame their misfortunes on western plots and covert meddling.

It's the old cold war "reds under the bed" syndrome, played out in reverse.

Mahmoud Ahmadinejad, Iran's president, fully shares Putin's paranoia. He has long decried supposed British and American plots to deny the Iranian nation its "rights" – assumed shorthand for a nuclear bomb.